I have no doubt that there was a slight difference in the paint on your car bumper... I too notice things like that. Some of us have a more discerning eye for minute shades of color, where others don't see it unless it is way off and obvious.... I recently had a stand-up row with a car repair shop that had repainted part of my car after a small collision. The bumper was a completely different shade of white compared to the rest of the car. Now I accept you can't get a perfect match in such circumstances but, to me, they were night and day different. He insisted they were a good match. Eventually, I consulted my wife and son who were with me and they said the guy was right - it was a match. I still look at the darn bumper every day and mutter darkly, but nobody else ever notices.
It's a fairly common result when paint is mixed in a small batch by a body shop. While some solid colors can be difficult to get a perfect match, many metallic colors are even tougher to match when mixed in small batches and only a couple body panels are re-painted.
The point I was making in my previous post, was that Honda OEM replacement body panels are painted with the same batch of paint as the body panels used on the assembly lines.